If you mean "representing values as a pair of integers P, Q, such that the value = P/Q", the answer is that they're very expensive to work with. You generally n

If you mean "representing values as a pair of integers P, Q, such that the value = P/Q", the answer is that they're very expensive to work with. You generally need to be using arbitrary-length integers for P and Q, because the values get very large very quickly. And as the values get large, operations like finding the lowest common multiple (required for almost any operation between two rationals) get expensive. As an example of the kinds of runaway increase in P and Q I'm talking about, consider calcuating the sum of x/(x+1) for values of x from 1 to 999, using python's fractions module: >>> sum(Fraction(x)/(x+1) for x in range(1000)) Fraction(7075502361382798268120793110090229940258706389605508780354900641905867818691614397718938054011886551220308728288024788414601821895503124242001568659978565050184167864774563757893457200995977901194872761682228965032590045181707202517783731540396927542180527680245723964874429994374505238893797078720320991788231820649513569449454767125309918934132672543174716802507966087781714014833406311821646176291230132112749836042537756095388477483, 7128865274665093053166384155714272920668358861885893040452001991154324087581111499476444151913871586911717817019575256512980264067621009251465871004305131072686268143200196609974862745937188343705015434452523739745298963145674982128236956232823794011068809262317708861979540791247754558049326475737829923352751796735248042463638051137034331214781746850878453485678021888075373249921995672056932029099390891687487672697950931603520000) That's a 436-digit P and a 433-digit Q. Rational types like this can definitely be a good choice in a use-case where accuracy is important, and operations do not compound. But they're a poor default, which is why we generally see float types as the most common non-integer number representation.